1. Field of the Invention
The present invention relates to a lithium secondary battery, and, more particularly, to a pouched lithium secondary battery with an improved structure so that an equal voltage is applied to a case and a positive electrode tab.
2. Description of the Related Art
In general, research into secondary batteries that are rechargeable, unlike primary batteries, has been intensified since the development of portable electronics such as digital cameras, cellular phones, notebook computers, etc. Secondary batteries include nickel-cadmium batteries, nickel-metal hybride batteries, nickel-hydrogen batteries, lithium secondary batteries, etc.
Lithium secondary batteries have a working voltage that is three times greater than nickel-cadmium batteries or nickel-metal hybride batteries, which typically have a working voltage of 3.6V or greater, and are commonly used as a power source of portable electronics. Lithium secondary batteries also have a greater energy density per unit weight. The recent increase in the usage of the lithium secondary batteries in portable electronics is attributed to these advantages.
Lithium secondary batteries may be classified into liquid electrolyte batteries and polymer electrolyte batteries, depending on the type of electrolyte used. In general, lithium secondary batteries which contain liquid electrolytes are called lithium ion batteries, and lithium secondary batteries which contain polymer electrolytes are called lithium polymer electrolytes.
Lithium secondary batteries may be manufactured in various shapes. Lithium ion batteries are typically manufactured in a cylindrical shape or a prismatic shape. Recently more attention has been focused on lithium polymer batteries that may be contained in a flexible pouch. Lithium polymer batteries are safe to operate, and are lightweight, so they are suitable for slim, lightweight portable electronic devices.
Referring to FIG. 1, a conventional pouched lithium secondary battery 10 includes a battery unit 11 and a case 12 that provides a space 12a for accommodating the battery unit 11.
The battery unit 11 is formed by rolling a stack of a positive electrode plate, a separator, and a negative electrode plate in one direction, or by repeatedly stacking a positive electrode plate, a separator, and a negative electrode plate. The positive electrode plate and the negative electrode plate of the battery unit 11 are electrically connected to a positive electrode tab 13 and a negative electrode tab 14, respectively.
An end of each of the positive electrode tab 13 and the negative electrode tab 14 goes through the sealing portion 12b of the case 12, and extends out of the case 12. The externally extended ends of the positive electrode tab 13 and the negative electrode tab 14 are connected to ports of a protective circuit board (not shown),
A portion of each of the positive electrode tab 13 and the negative electrode tab 14 located in the sealing portion 12b of the case 12 is wound with a sealing tape 15 to prevent an electrical short circuit.
The case 12 is a pouch comprised of an intermediate metallic foil and internal and external insulating films attached to opposite surfaces of the intermediate metallic foil. This pouched case is very flexible.
The case 12 has the space 12a for accommodating the battery unit 11, as described above. The sealing portion 12b, which may be thermally fused, is formed around the edge of the space 12a. 
FIG. 2A is an enlarged, sectional view taken along line I-I in FIG. 1. FIG. 2B is an enlarged, partial view of FIG. 2A.
Referring to FIGS. 2A and 2B, the case 12 is made of a complex film comprised of an intermediate layer 12c that is made of a metallic foil, for example, an aluminum foil, and an internal layer 12d and an external layer 12e, which are protective layers attached to opposite surfaces of the intermediate layer 12c. 
The battery unit 11 that includes a roll of the positive electrode plate 11a, the separator 11c, and the negative electrode plate 11b, which are stacked upon one another, is accommodated in the space 12a in the case 12. The positive electrode tab 13 and the negative electrode tab 14 are drawn out from the positive electrode plate 11a and the negative electrode plate 11b, respectively, as shown in FIG. 1. As described above, an end of each of the positive electrode tab 13 and the negative electrode tab 14 is exposed outside the case 12, and a portion of each of the positive electrode tab 13 and the negative electrode tab 14 located in the sealing portion 12b is wound with a sealing tape 15.
In the manufacture of the pouched lithium secondary battery 10 having the structure described above, the positive electrode plate 11a and the negative electrode plate 11b are electrically connected to the positive electrode tab 13 and the negative electrode tab 14, respectively, and a stack of the positive electrode plate 11a, the separator 11c, and the negative electrode plate 11b is rolled to form the battery unit 11.
The battery unit 11 is positioned in the space 12a of the case 12 after an extending process such that an end of each of the positive electrode tab 13 and the negative electrode tab 14 is exposed outside the case 12.
In this state, predetermined amounts of heat and pressure are applied to the sealing portion 12b of the case 12 to complete the manufacture of the pouched lithium secondary battery 10. The pouched lithium secondary battery 10 is subjected to a series of formation processes, including charging, aging, discharging, etc., to structurally stabilize the pouched lithium secondary battery 10 and to determine whether it operates properly.
However, the conventional pouched lithium secondary battery 10 has the following problems.
When a small crack occurs in the internal layer 12d when shaping the case 12, which is a complex film comprised of the insulating internal layer 12d, the metallic intermediate layer 12c, and the insulating external layer 12e, the positive electrode tab 13 and the negative electrode tab 14 may be electrically connected to the intermediate layer 12c when operating the battery 10.
This is especially problematic when the negative electrode tab 14, which is made of a nickel plate, contacts the intermediate layer 12c, which is made of aluminum, due to damage of the internal layer 12d, because the case 12 then has a negative potential. As a result, the intermediate layer 12c, which cannot be protected by the internal layer 12d any longer, corrodes due to a potential difference between the intermediate layer 12c and lithium ions that come from an electrolyte and adhere to the surface of the intermediate layer 12c as the battery is operated.
As the case 12 corrodes more and more, the intermediate layer 12c decomposes, forming a gap in the case 12 and allowing external air to enter through the gap into the case 12.
As moist air enters the case 12, moisture in the air reacts with the battery unit 11, which is charged, particularly, with the negative electrode plate 11b of the battery unit 11, and generates gases. The generated gases may swell the case 12 and open a sealed portion 12b of the battery pack of the lithium secondary battery 10.
Therefore, it is necessary to check whether there is an electrical contact between the intermediate layer 12c and the negative electrode tab 14, due to damage of the internal layer 12d, in a formation process after the manufacture of the lithium secondary battery 10.
However, even when the pouched lithium secondary battery 10 is seriously corroded and swollen due to the electrical contact between the intermediate layer 12c and the negative electrode tab 14, the pouched lithium secondary battery 10 may be determined to be in a normally charged state from its voltage level measured in a formation process. In other words, it is not possible to accurately determine whether the pouched lithium secondary battery 10 functions properly or not. Furthermore, the swelling of the lithium secondary battery 10 leads to deformation of the battery pack.